Evaluation of the Ability of the Seven29 Array to Measure Jaw Speeds On a TomoTherapy-System

Purpose: To evaluate, if the PTW array Seven29 is a suitable tool to measure the speed of moving jaws on a TomoTherapy-System.

Methods:The PTW Octavius II Phantom is placed on the treatment couch with the Seven29 array in a coronal orientation. Since the update of the System to deliver radiation with moving jaws has not been installed yet, the movement is simulated by moving the phantom thru a stationary beam. The signals of nine ionization chambers of the array, with a distance of 1cm in the direction of couch travel, are recorded every 200ms. To measure the speed of the array relative to the jaws, the penumbra of each jaw moving across each ion chamber is evaluated. For a programmed couch speed of 1mm/s the relative speeds of the jaws are calculated over the complete distance of 8cm as well as for each sub-distance of 1cm. Additionally, the same measurements are taken with a variation of the programmed couch speed of +/-1%.

Results:The speed for the back jaw (IEC-Y positive) was 1.0025mm/s while the speed of the front jaw (IEC-Y negative) was 1.0018mm/s. For the sub-distances the standard deviations were 0.001mm/s and 0.0019mm/s respectively. For the procedures with a programmed speed of +1% and -1% the results for the back jaw were 1.0125mm/s and 0.9926mm/s respectively. For the front jaw it was 1.0118mm/s and 0.9919mm/s.

Conclusion:This work shows a method to measure the speed of moving jaws by using time-resolved measurements with a Seven29 array by PTW. This method will be important for quality assurance and the commissioning of the new TomoEDGE functionality. The use of the seven29 array along with the Octavius II phantom is very effective, since the same setup can be used for the verification of patient treatment plans.

Funding Support, Disclosures, and Conflict of Interest: The department receives research funding from Accury Inc.